The present invention relates to a black and white photographic material and in particular to a black and white photographic material suitable for use in optical contact copying. The invention also relates to a method of manufacturing a printed circuit board using the black and white photographic material.
Black and white high contrast silver halide materials are used widely as originals for optical contact copying onto other photosensitive materials. For example in the printing industry, page separations are exposed by imagesetter onto film which is then copied (exposed) onto printing plates by ultra-violet (UV) contact exposure. The exposed printing plate is then processed to produce an ink-receptive image for printing on a press. Sometimes, pages are physically assembled by cutting and pasting images and text from various sources. The assembled page may then be copied by camera onto another sheet of film. This process also induces a feature size change before the final copying of this film image onto a printing plate which further exaggerates the size gain.
Another example of this type of process is the manufacture of printed circuit boards (PCBs) where electronic circuit track layouts are exposed by photoplotter onto film intermediates, called phototools or photomasks. FIG. 1 shows an arrangement for exposing an image through a photomask onto the circuit board. The photomask comprises a support 1 and an image layer 2 arranged thereon. The photomask is positioned immediately adjacent to a copper-clad, resist-covered PCB substrate on a contact-copying frame (not shown). To improve the intimacy of contact it is normal practice to evacuate air from the contact frame. Incident radiation such as UV light is provided by a source (not shown) within the contact frame and is received by the photomask and transmitted directly to the photoresist layer 4 through openings in the mask. The mask is a photographic film and openings in the mask correspond to minimum density regions of the image on the film.
The transmitted radiation causes a change in the properties of the photoresist, e.g., a hardening such that areas exposed to the radiation are physically different from those that have been hidden by the mask layer 2. After exposure the photoresist is processed to remove it from areas where it is desired to etch away the copper such that the resultant structure has regions of copper exposed and regions concealed by photoresist. The exposed areas of the copper are then etched. After etching, the remaining photoresist is removed from the PCB to reveal the track pattern.
A problem, known as contact gain, that exists with these kind of contact copying processes is that the image feature size on the copy is often slightly different from the feature size on the original mask. In the printing industry this effect is known as xe2x80x9cdot gainxe2x80x9d, a typical measure of which is the % transmittance change of a black area, often a dot, within its total possible area, defined by the screen ruling. Thus if a dot obscuring 50% of the possible area it can occupy grows to 55% (or 45% depending on whether the printing plate is positive or negative working) on the copy, a dot gain of 5% has occurred.
In the PCB industry, contact gain can affect the width of features, e.g. tracks or lines, on a PCB and is referred to as xe2x80x9cline width gainxe2x80x9d. Thus, if a 100 xcexcm line becomes a 105 xcexcm line on the photoresist, a line width gain of 5 xcexcm would have occurred. In the example shown in FIG. 1, the regions of the negative-working photoresist 4 that have been exposed to the UV radiation are slightly larger than the corresponding openings in the mask 2 (where the contact gain is defined as the difference in the feature size of the copy from that of the original).
In addition to the problem of contact gain, it is known that the amount of contact gain may vary according to the position of any particular image point across the contact copying frame.
These gain effects make process control more difficult in both examples discussed above. The problem is becoming more severe as the average feature size to be copied is decreasing due to the drive to reduce track sizes on PCBs and to reduce screen ruling or to use stochastic screening techniques with very small dot size in the printing industry.
A further development in the printed circuit board industry is the use of dry film resist (as shown in FIG. 5) rather than the conventional liquid resist where the resist is applied directly to the substrate. As will be explained below, dry film resist is supplied as a roll comprising 3 layers: a carrier layer, a thin transparent support layer and a photoresist layer. The carrier layer is normally separated from the other 2 layers when the dry film resist is applied to the circuit board substrate. The photoresist support layer, for example a 20 xcexcm thick Mylar(trademark) film (polyethylene terephthalate) or any other UV transparent material, is placed uppermost on the circuit board with the photoresist layer directly on top of the copper. Thus the Mylar(trademark) film separates the photomask and the photoresist. Given that feature sizes in printed circuit boards may now be approaching the thickness of the Mylar(trademark) film, this separation represents a significant distance and further degrades the contact performance by increasing the contact gain of the system in comparison to use of liquid resists, where no support layer is necessary.
It is an aim of the present invention to reduce the amount of contact gain through improved design of the silver halide film original and thus facilitate process control.
It is also an aim of the present invention to provide a photographic material suitable for use in the manufacture of PCBs or the production of printing plates, adapted such that contact gain is minimized.
According to a first aspect of the present invention, there is provided a black and white silver halide material for optical contact copying, comprising: at least one photographic emulsion layer including a silver-halide-containing matrix in which the matrix includes a polymer and a hydrophilic binder. The ratio of the weight of silver in the emulsion layer to the weight of polymer in the emulsion layer per unit area is less than 2.0.
Preferably, the hydrophilic binder is gelatin. The polymer may be a polymer derived from the polymerization of one or more ethylenically unsaturated monomers. Preferably, the polymer is selected from a group consisting of acrylates, methacrylates, acrylamides and methacrylamides.
According to a second aspect of the present invention, there is provided a method of optical contact copying comprising the step of irradiating a substrate onto which a pattern is to be copied with radiation through an optical mask of the pattern, such that regions of the substrate are selectively exposed or hidden from the radiation. The mask is formed of a black and white photographic material according to the first aspect of the present invention.
It has been found that the amount of contact gain may be controlled by minimizing the coated weight of silver consistent with maintaining adequate maximum image density, together with maximizing the coated weight of polymer contained in the emulsion layer consistent with high quality manufacturing requirements.